Photoimageable compositions useful as resists for forming printed circuits, printing plate, solder masks or the like have been used for some time now. The earliest photoresists were solvent-borne and solvent-developable. The development of aqueous-developable resists represented an advance in reducing solvent emissions in the immediate workplace and general environment. The continued emphasis in reducing organic solvent both from the workplace and from the general environment has spurred the search for waterborne photoresists which are formulated and applied as aqueous liquids.
U.S. Pat. No. 5,045,435, the teachings of which are incorporated herein by reference, describes a waterborne photoresist composition which is developable in alkaline aqueous solution. The composition comprises a multifunctional monomer, a photoinitiator and a latex of water-insoluble carboxylated acrylic copolymer. To stabilize the composition and to adjust the viscosity, this patent teaches neutralizing the latex polymer to at least 25% with a base, such as ammonia, another amine, or sodium hydroxide.
The high degree of neutralization required in the composition in U.S. Pat. No. 5,045,435 has several disadvantages. The neutralizer, e.g., amine, reacts with the acid functional polymer, producing water soluble salts. Because all of the neutralizer is not removed during drying, the remaining salts become sites for potentially damaging chemical attack. It is therefore desirable to reduce the amount of neutralizers to improve chemical resistance. Amines or ammonia, when used as the neutralizer, have associated odors, amines having a characteristic "fishy" odor. Primary amine neutralizers and ammonia are highly reactive, resulting in viscosity drift in the lacquer prior to coating, the viscosity drifting as much as 3000 cps over one week.
In teaching and demonstrating the need for substantial neutralization in its compositions, U.S. Pat. No. 5,045,435 also teaches that obtaining the critical rheology cannot be obtained with the addition of thickening agents.
It is a general object of the present invention to provide a waterborne photoimageable composition requiring a substantially reduced amount of neutralization of the latex polymer.